Alloy for gas reformer tubes



United States Patent US. Cl. 75171 Claims ABSTRACT OF THE DISCLOSURE Heat and corrosion resistant alloy containing nickel, chromium, tungsten and carbon, along with titanium and/or zirconium, has characteristics especially suitable for use in gas reforming plant apparatus.

The present invention relates to heat resistant alloys and, more particularly, to nickel-chromium-tungsten alloys for use at elevated temperatures.

It is well known that many industrial processes are now operated at temperatures of up to 1000 C. and above and require plant apparatus made of alloys that will withstand stress and corrosion at these temperatures. Alloys for such purposes should also have good casting properties, including satisfactory castability in air, so that they can be readily made into components of the plant by casting and, in addition, should also be weldable.

One important industrial process that requires heat and corrosion resistant plant apparatus is the reforming of hydrocarbons by reaction with steam in the presence of a catalyst to form a mixture of hydrogen and carbon monoxide. This gas mitxure is widely used under the name of synthesis gas for the synthesis of alcohol and is also used as a source of hydrogen for ammonia production. The reforming reaction is carried out at high temperatures, usually in the range of 800 C. to 1000 C., under elevated pressure in tubular reactors, known as reformer tubes, which are heated externally by the combusion of hydrocarbon fuels.

Alloys for reformer tubes must have good resistance to creep and rupture under stress at the operating temperature and must maintain adequate toughness when heated for prolonged periods at elevated temperatures. Also required are good resistance to carburization and corrosion by the combustion products of impure fuels, which give rise to sulphidation attack on the alloys. Hitherto reformer tubes have usually been made by centrifugal casting from 25% chromium-20% nickel steels known by the Alloy Casting Institute designation HK, but there is a need for commercially economical alloys having superior stress-rupture strength to the HK steels while maintaining satisfactory corrosion resistance. Of coure, a number of alloys having high stress-rupture strength have been developed for use in small articles such as turbine buckets. However, in the field of large plant apparatus such as gas reforming apparatus, economic and practical considerations are very different than in fields such as the gas turbine field and many alloys made for gas turbines wouldbe prohibitively expensive to produce for large plant apparatus. Moreover, most such alloys contain titanium or aluminum or both in such proportions that they cannot satisfactorily be cast in air, and are therefore unsuitable for making large cast articles and parts for which air-casting is the only practical method of manufacturing. Thus, a very important factor in the present problem is to obtain the required characteristics without requiring use of elements such as cobalt, columbium and tantalum and without necessity 3,508,917 Patented Apr. 28, 1970 for unusually expensive production or commercially impractical techniques, e.g., melting and casting under vacuum.

Although many attempts were made to overcome the foregoing difiiculties and other difliculties and disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

There has now been discovered a new alloy having a specially balanced composition providing a new combination of characteristics that are especially suitable for production and use of cast reformer tubes and other plant products and apparatus that are subjected in use to similar conditions of stress, temperature and corrosive attack.

It is an object of the present invention to provide a new alloy composition.

It is a further object of the invention to provide an alloy having a combination of characteristics including resistance to corrosion by fuel combustion products, good stress-rupture strength at high elevated temperatures, adequate toughness when subjected to prolonged service at elevated temperatures and satisfactory castability and weldability.

Another object of the invention is to provide heat and corrosion resistant alloy products and articles, including products and articles for use in gas plant apparatus and particularly including cast reformer tubes.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention is directed to an alloy containing 16.5% to about 32% chromium, 16% to about 36% tungsten, 0.15% to 0.9% carbon, 0.8% to about 5% of metal from the group consisting of up to 2.5% titanium, up to 5% zirconium and mixtures thereof in amounts such that 3 percent (Ti) +2 (percent Zr) equal at least 2.4%, and 3 (percent Ti) +2 (percent Zr) equal not more than 5% and with the balance essentially nickel. Molybdenum is not an equivalent for tungsten in the alloy of the present invention and replacement of tungsten by an equal atomic percentage of molybdenum detrimentally reduces the stress-rupture strentgh of the alloy. Of course, the balance of essentially nickel in the alloy of the invention can include minor amounts of impurities, including residual deoxidizers, e.g., up to 3% or even 5% iron and up to 1% or even 2% individually of silicon and manganese, but advantageously the impurity content of the alloy is maintained as low as practical. In practice, it is advantageous to maintain the content of silicon below 1% in order to obtain the highest resistance to corrosion and stressrupture life and to maintain the content of manganese below 1% if it is employed as a deoxidant and below 0.5% if deoxidation is effected by other means, e.g., by addition of calcium or magnesium. Thus, for obtaining all the required results of the invention the total percentage of essentials nickel, chromium, tungsten, carbon and metal from the group of titanium and zirconium is at least about and is advantageousl 98% or higher. It is an advantage of the present alloys that they are composed of only a few essential elements, the main hardening effect being due to tungsten. Other elements such as molybdenum, niobium, tantalum, aluminum and cobalt are not deliberately added. If they were present in amounts greater than would be introduced as impurities from the raw materials or from scrap employed as part of the melting charge, they could upset the required balance between the proportions of the essential elements and thereby impair the high-temperature properties of the alloys. Although they may be tolerated as impurities in a total amount not exceeding about 1%, they are desirably entirely absent.

Molybdenum in greater than impurity amounts leads to loss of stress-rupture life at elevated temperatures and of impact strength after prolonged heating at high temperatures, e.g., 850 C. The presence of aluminum leads to the formation of oxide inclusions in air castings and consequent unsoundness and weakening of the castings. Aluminum is also detrimental to the impact strength of the alloys after prolonged heating at 850 C. Niobium and tantalum have simliar disadvantages to those of molybdenum. Also, the alloy can contain as residual deoxidants up to about 0.05% in total of one or both of calcium and magnesium. All compositional percentages referred to herein are by Wei ght.

Important virtues of the subject alloy include high strength and good resistance to corrosion at elevated temperatures of the order of 900 C. and 1000 C. and higher, e.g., 1010 C. and 1050 C. At temperatures of around 1000 C. the alloy exhibits an exceptionally good combination of stress-rupture strength with resistance to corrosion. For instance, at 1010 C. the alloy is characterized by a stress-rupture life of at least about hours at a stress of 3.5 long tons per square inch (t.s.i.) when in the as-cast condition. The good elevated-temperature corrosion resistance of the alloy includes not only oxidation resistance but also resistance to sulphidation and carburization. Of further advantage, the alloy can be melted and cast either in air or vacuum to obtain satisfactory results and can be joined by welding, e.g., welded by the T16 method. Also, the alloy has adequate impact strength and good resistance to embrittlement. The good air melting and casting characteristics of the alloy are particularly useful where the alloy is required in a product form that is difiicult or impractical to produce in vacuum, e.g., where the required product of the alloy is a centrifugally cast tube.

In carrying the invention into practice it is advantageous that the alloy contain 18.5% to 23% chromium, 20% to 28% tungsten, 0.15% to 0.6% carbon, 0.8% to 2.5 titanium and up to 2.5% zirconium with the amounts of titanium and zirconium correlated to provide that the sum of the titanium content plus the zirconium content is not greater than 3.5% and with the balance essentially nickel in order to obtain advantageously high stress-rupture life of at least 60 hours at 1010 C. and 3.5 t.s.i. and to also obtain advantageously good characteristics of castability into sound castings and freedom from embrittlement on prolonged exposure to elevated temperatures.

For the purpose of giving those skilled in the art a better understanding and appreciation of the advantages of the invention the following illustrative examples (referred to hereinafter as Alloys 1 through 20) of the invention are given.

Chemical compositions and test results pertaining to Alloys 1 through 20, as set forth in the following Tables I through V hereinafter, illustrate the good stress-rupture strength, corrosion resistance and impact strength of the new alloy. Additionally, to further illustrate advantages of the invention in comparison with other alloy compositions, test results pertaining to a number of other alloy compositions( referred to as Alloys A through L), which are not in accordance with the invention, are also provided hereinafter.

In order to attain the herein provided combination of needed characteristics for reformer tube alloys, it is important for all the constituents of the alloy to be within the ranges specified for the invention. If chromium is less than 16.5% the resistance to corrosion by fuel ash is poor. Both corrosion resistance and stress-rupture strength are advantageously high when the chromium in the alloy is from 18.5% to 23% but above 23% chromium both of these properties fall off appreciably (although still remaining satisfactory up to 32% chromium) and are unsatisfactory above 32% chromium. Such effects are illus- 4 trated by the test results set forth in Table I, which results pertain to cast specimens of alloys melted and cast under vacuum, nominally containing 25% tungsten, 0.4 carbon, 1% titanium, 1% zirconium, the indicated varied amount of chromium with the balance essentially nickel and which were subjected to stress-rupture tests and also to elevated temperature corrosion tests. For the corrosion test referred to in tables I through IV, specmens of the alloys were half immersed in a molten mixture of 25% sodium chloride and 75% sodium sulfate for different periods of time and at different temperatures as set forth in the tables. These tests simulated attack by fuel ash and the loss in Weight is an inverse measure of the re sistance of the tested alloy to fuel-ash corrosion. Corrosion test results set forth in Table I show that Alloys A, B and C, which did not contain amounts of chromium in accordance with the invention, suffered unsatisfactorily high weight loss due to corrosion whereas Alloys 1 through 6 of the present invention exhibited satisfactory corrosion resistance.

TABLE I Stress-Rupture at 1010 C. and 3.5 t.s.i.

Life, hr.

16 hrs. at 900 C.

16 hrs.

percent T.s.l.=Long tons (2,240 pounds) per square inch. Elong. (percent) =Percent elongation in 1.25 inch gage length. Mg./cm. =Milligrams per square centimeter.

Hr.=Hours.

Tungsten makes an important contribution to the stressrupture strength of the alloy and for this purpose at least 16% tungsten must be present. The stress-rupture strength increases, reaching a peak at about 25% tungsten, and then decreases as the tungsten content increases further. The tungsten content should accordingly not exceed 30% and is advantageously 20% to 28% for obtaining particularly high stress-rupture strength.

Results with varied tungsten contents are illustrated in Table II which shows results obtained with cast specimens of alloys melted and cast under vacuum and containing nominally 20% chromium, 0.4% carbon, 1% titanium, 1% zirconium, the varied indicated amounts of tungsten and the balance essentially nickel. Alloys 7, 2 and 8 had satisfactory stress-rupture life and corrosion resistance whereas Alloys D and E, which did not have sufficient tungsten for the present invention, had unsatisfactorily low stress-rupture life.

TABLE II Stress- Rupture at 10130; and Weight loss (mg/crud) .S.l. 16 hrs. 300 hrs. 16 hrs. Life, Elong., at 900 at 900 at 1,050 hr. percent 0. C. C.

The stress-rupture strength of the subject alloy generally increases as the carbon content is increased above 0.15%. However, if the carbon content is reduced below 0.15% the stress-rupture strength and corrosion resistance fall off detrimentally and if the carbon content is increased above 0.9% the alloy becomes unsatisfactorily brittle after heating for 1000 hours at elevated temperatures such as 850 C. Advantageously for good stress-rupture strength and resistance to corrosion and embrittlement, the carbon content is from 0.15 to 0.6%. Table III shows results Weight loss (mg/0111 300 hrs. at 900 C.

6 of stress-rupture, corrosion and impact tests carried out vacuum and nominally contained 25% tungsten, 0.4% upon cast specimens of alloys melted and cast under carbon and the indicated different amounts of titanium vacuum and containing nominally 20% chromium, 25 and zirconium. In addition, Alloys 2, 12 through 16 and tungsten, 1% titanium, 1% zirconium, the indicated var- G through K contained nominally 20% chromium while ied amounts of carbon and the balance essentially nickel. 5 Alloys 17 and L contained 18% chromium. The balance Impact results in Table III pertain to alloys which had in each of these instances was nickel. From Table IV it is been heated 1000 hours at 850 C. Results of tests with apparent that satisfactory rupture life with good corro- Alloys 2, and 11 illustrate advantageously good rupsion resistance were obtained with alloys of the invention ture strength of at least 100 hours at 3.5 t.s.i. and 1010 having a variety of percentages of titanium and/or zir- C. accompanied by adequate impact strength after pro- 10 conium in accordance with the invention whereas Alloys longed elevated temperature exposure, which were ob- G through L, which did not contain amounts of titanium tained with alloys of the invention containing at least and/or zirconium in accordance with the invention, did about 0.4% carbon, e.g., about 0.4% to 0.8% carbon. not have a satisfactory combination of rupture life and Results pertaining to Alloy F wherein the carbon content corrosion resistance.

TABLE IV Stress-rupture at 1,010 O. and 3.5 t.s.i. Weight loss (mg/cm!) Elong., 16 hrs. at 300 hrs. at 16 hrs. at Alloy Ti, percent Zr, percent Life, hr. percent 900 0. 900 C. 1050 C 1.1 63 39 13 19 18 73 48 12 51 44 141 52 10 43 25 72 40 5 13 5 156 35 s 26 is 116 28 12 29 36 46 28 33 210 21 44 30 14 147 12 56 49 5s 50 60 33 26 223 12 56 47 5 36 16 310 780 is too low for the present invention show that long-time An important feature of the present invention is that corrosion resistance is unsatisfactorily low when an alloy good results are obtained with air melted alloys in accordcontaining nickel, chromium, tungsten, titanium and zirance herewith. When the alloys are melted in air they can conium in accordance with the invention does not conbe deoxidized by additions of silicon and manganese and tain sufiicient carbon for the invention. are advantageously deoxidized with calcium of magnesium TABLEfiIII Stress-rupture at 1010 C. and 3.5 t.s.i. Weight loss (mg/0111.

Impact Elong., 16 hrs. at 300 hrs. at 16 hrs. at strength Life, hr. percent 900 0. 900 C. 1050 C. (IL-lbs.)

Impact strength (ft.lbs.) Charpy impact energy in foot-pounds.

Titanium and zirconium enhance both stress-rupture to better obtain good rupture strength. Table V shows strength and corrosion resistance. For these objects, titasatisfactory results of stress rupture tests on cast specimens nium is more effective than zirconium. Thus, in the abof the alloys that were melted and cast in air and had the sence of zirconium, at least 0.8% titanium is required chemical compositions set forth in the following Table V.

TABLE v Stress-rupture at 1,010 C. and 3.5 t.s.i.

, W, Cr, Ti, Zr, Elong. Alloy percent percent percent percent percent Ni Life ,hr. percent Deox 1s 0.44 24. 4 20. 2 0. 9 0. 5o Bal- 64 47 1)' 0. 42 24. 4 19. 8 0. 95 0. 47 Bal. 92 31 (2) 0. 42 24. 6 20. 0 0. 95 0. 44 B21. 72 26 (3) Deox.=De0xidation Additions of: (1) 0.3% silicon+0.3% manganese, (2) 0.03% calcium as calcium silicide, (3) 0.03% magnesium as nickel-magnesium.

al.=Balance (including any residual deoxldizers).

while in the absence of titanium, at least 1.2% zirconium In addition to providing the new alloy described herein must be present, so that 3 (percent Ti) plus 2 (percent the present invention also provides, as a new article of Zr) is at least 2.4%. On the other hand excessive amounts manufacture, a cast reformer tube COIHPOSed 0f the alloy f ith i i o zirconium, i e o th 25% of the invention. Also, the invention is applicable in the titanium or more than 5% zirconium, lead to poor cast- Production of Othfif articles for gas Plant apparatus cluding supports for reformer tubes. Furthermore, the

ing properties, particularly when the alloys are melted sub ect alloy is also useful for other heat and corrosion in air, and for the best combination of stress-rupture and casting properties the alloys advantageously contain j i F F at least 0.8% titanium together with zirconium in an 1 s g 5 consist. S 11 f 16 t b t amount not exceeding that of titanium. The total amount y mg es en la y o a o a on 32% chromium, 16% to about 30% tungsten, 0.15% to 0.9% carbon, metal from the group consisting of up to 2.5% titanium, up to 5% zirconium and mixtures thereof of titanium and zirconium advantageously does not exceed 3.5% and is more advantageously not greater than 2.5 in amounts such that 3 (percent Ti) plus 2 (percent Zr) Results obtamed w1th varying titanium and zirconium equal t least 2.4%, and 3 (percent Ti) plus 2 (percent contents are set forth in Table IV. These results pertain Zr) equal not more than 5% and with the balance essento specimens of alloys that were melted and cast under tially nickel.

2. An alloy as set forth in claim 1 containing 18.5% to 23% chromium.

3. An alloy as set forth in claim 1 containing 20% to 28% tungsten.

4. An alloy as set forth in claim 1 containing 0.15% to 0.6% carbon.

5. An alloy as set forth in claim 1 wherein the sum of titanium and zirconium does not exceed 2.5%.

6. An alloy as set forth in claim 1 containing both titanium and zirconium with the titanium being present in an amount not less than 0.8% and with the zirconium content not exceeding the titanium content.

7. A cast article composed of the alloy in claim 1 and characterized by a stress rupture life of at least about 20 hours at 3.5 long tons per square inch at 1010 C.

8. A cast reformer tube made of the alloy as set forth in claim 1.

9, An alloy as set forth in claim 1 containing 18.5% to 23% chromium, to 28% tungsten, 0.15% to 0.6% carbon, 08% to 2.5% titanium and up to 2.5% zirconium With the amounts of titanium and zirconium correlated to provide that the sum of the titanium content plus the zirconium content is not greater than 3.5%.

10.- A cast article composed of the alloy set forth in claim 9 and characterized by a stress-rupture life of at least about hours at 3.5 long tons per square inch at 1010 C.

References Cited UNITED STATES PATENTS 3,127,265 3/1964 Avery 171 3,164,465 1/1965 Thielernann 75l71 3,293,030 12/1966 Child et al 75171 RICHARD O. DEAN, Primary Examiner 3,508,917 April 28, 1970 Patent No. Dated Inventor) MICHAEL JOHN FLEE'IWOOD & ALFRED JOHN FLETCHER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r Column 1, line 55, insert apostrophe marks 1 around HK.

Column 2, line 33, for "36%" read -30%-; lines 36, 37 and 38 for "3 percent (Ti) +2 (percent Zr) equal not more than 5%" read A -3 Ti) 2 Zr) equal at least 2.4%, and

Ti) Zr) equal not more than 5%--; and line 57 insert "the" after the word -of-.

Column 5, Table III, line 41, "16 hrs. at 1050C." column, for "28" read -l8-.

Column 6, line 33 for "of" read -or--; lines 73 and 74 for "3 (percent Ti) plus 2 (percent Zr) equal not more than 5%" read Ti) Zr) equal not more than 5%.

Signed and sealed this 29th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

